The present invention relates to two surprisingly simple processes for the size-selective preparation of soluble metal colloids and of metal clusters fixed to supports. The invention also includes the preparation of bimetallic colloids and bimetallic clusters fixed to supports. Nanostructured metal colloids or clusters, especially in a range of sizes of 1 to 10 nm, are known to be useful catalysts. It has further long been known that the reduction of transition metal salts results in insoluble metal powders unless the reduction is performed in the presence of stabilizers which wrap around the intermediary nanometer-sized metal clusters and protect them against undesirable agglomeration [G. Schmid, Clusters and Colloids, VCH, Weinheim, 1994; B. C. Gates, L. Guczi, H. Knozinger, Metal Clusters in Catalysis, Elsevier, Amsterdam, 1986]. The stabilizers which have become known as yet include special ligands, such as triarylphosphanes, polymers, such as polyvinylpyrrolidones, surfactants, such as long-chain tetraalkylammonium salts (R.sub.4 N.sup.+ X.sup.-), and in some cases special solvents. The reductants required for the reduction of the metal salts include, e.g., hydrogen, hydrazine, formaldehyde and various boron hydrides [ref.: see above]. Recently, the first electrochemical methods for the preparation of tetraalkylammonium salt stabilized metal colloids and their fixing to supports have been described [M. T. Reetz, W.Helbig, J. Am. Chem. Soc. 116 (1994), 7491; M. T. Reetz, S. A. Quaiser, Angew.Chem. 107 (1995), 2461, Angew. Chem. Int. Ed. Engl. 34 (1995), 2240]. Thus, a metallic sacrificial anode (e.g., a Pd sheet) is used as the metal source. In the presence of the conducting salt R.sub.4 N.sup.+ X.sup.-, the metal sheet dissolves through anodic dissolution, the generated metal salts migrating to the cathode where they are reduced again. The metal atoms aggregate into nanostructured metal colloids stabilized by the tetraalkylammonium salts. Alternatively, two inert electrodes may be used in the electrochemical method, a transition metal salt serving as the metal source, i.e., the metal salts are directly reduced electrochemically in the presence of R.sub.4 N.sup.+ X.sup.-. An essential advantage of this method is the fact that the size of the nanostructured R.sub.4 N.sup.+ X.sup.- stabilized clusters can be varied in a well-aimed manner by adjusting the current density. This is important because the size of metal clusters is known to have a strong influence on its catalytic properties [G. Schmid, Clusters and Colloids, VCH, Weinheim, 1994]. Indeed, the control of the cluster size is considered the greatest challenge in this field [J. S. Bradley, in Clusters and Colloids (G. Schmid, ed.), VCH, Weinheim, 1994, p. 490].
The disadvantages of the above mentioned methods include: 1) the high costs of some reductants, or their, in part, difficult handling, as in the case of hydrogen which involves a danger of explosion and specific and expensive handling methods; 2) lack of size selectivity; 3) complicated separation of reductants or side-products; 4) impure products from partial incorporation of the reductants (e.g., hydrogen or boron); and/or 5) use of expensive stabilizers, such as phosphanes or tetraalkylammonium salts.
In addition to the chemical and electrochemical reduction of transition metal salts in the presence of the above mentioned stabilizers, some metal colloids can also be prepared using metal vaporization [S. C. Davis, K. J. Klabunde, Chem. Rev.82 (1982), 153; K. J. Klabunde, G. Cardenas-Trivino, in Active Metals: Preparation, Characterization, Applications (A. Furstner, ed.), VCH, Weinheim, 1996, p. 237]. Thus, a transition metal is vaporized, and the metal vapor is introduced into a cold matrix consisting of a solvent. In some cases, especially when polar solvents such as tetrahydrofurane or acetone are used, the metal colloid solutions generated at low temperatures could be brought to room temperature without an undesirable agglomerization of the nanostructured metal clusters occurring. This is due to solvent stabilization. Some of the thus prepared solvent-stabilized metal colloids have been employed as catalysts in hydrogenations. Thus, this method circumvents the above mentioned drawbacks. However, metal vaporization is an expensive method because complicated devices and a high expenditure of energy are required. The size selectivity on a preparatory scale is also problematic.
If hydrogen is used in special solvents, such as propylene carbonate, for the reduction of Pd salts, as in the in-situ hydrogenation of fatty acids, then solvent-stabilized Pd clusters are involved as hydrogenation catalysts [A. Behr, H. Schmidke, Chem. Ing. Tech. 65 (1993) 568; A. Behr, N. Doring, S. Durowicz-Heil, B. Ellenberg, C. Kozik, C. Lohr, H. Schmidke, Fett Wiss. Technol. 95 (1993) 2]. Size selectivity is not possible, however.
Another method relates to the simple thermolysis of certain transition metal salts in methyl isobutyl ketone as the solvent and stabilizer. The thermolysis of Pd salts in this medium could be used as an example to show that this solvent stabilizes Pd clusters. However, the Pd clusters are relatively large, i.e., larger than 8 nm, and further, a control of the cluster size, i.e., size selectivity, is not possible [K. Esumi, T. Tano, K. Meguro, Langmuir 5 (1989), 268].